It is widely accepted that Earth's early atmosphere contained less than 0.001 per cent of the present-day atmospheric oxygen (O-2) level, until the Great Oxidation Event resulted in a major rise in O-2 concentration about 2.4 billion years ago(1). There are multiple lines of evidence for low O-2 concentrations on early Earth, but all previous observations relate to the composition of the lower atmosphere(2) in the Archaean era; to date no method has been developed to sample the Archaean upper atmosphere. We have extracted fossil micrometeorites from limestone sedimentary rock that had accumulated slowly 2.7 billion years ago before being preserved in Australia's Pilbara region. We propose that these micrometeorites formed when sand-sized particles entered Earth's atmosphere and melted at altitudes of about 75 to 90 kilometres (given an atmospheric density similar to that of today(3)). Here we show that the FeNi metal in the resulting cosmic spherules was oxidized while molten, and quench-crystallized to form spheres of interlocking dendritic crystals primarily of magnetite (Fe3O4), with wstit(FeO)+metal preserved in a few particles. Our model of atmospheric micrometeorite oxidation suggests that Archaean upper-atmosphere oxygen concentrations may have been close to those of the present-day Earth, and that the ratio of oxygen to carbon monoxide was sufficiently high to prevent noticeable inhibition of oxidation by carbon monoxide. The anomalous sulfur isotope (Delta S-33) signature of pyrite (FeS2) in seafloor sediments from this period, which requires an anoxic surface environment(4), implies that there may have been minimal mixing between the upper and lower atmosphere during the Archaean.